• Line baking sheets with silicone baking mats or parchment paper and set aside.

Make the sugar cookie dough.

• Place the flour, both sugars, salt, baking soda, and cream of tartar in the bowl of a food processor and process just until mixed.
• Cut the butter into small pieces and distribute over the flour mixture. Process until the dough resembles coarse meal, about 30 seconds.
• Stir the egg and vanilla together in a cup. With the processor running on low speed, pour the egg mixture through the feed tube and process until the dough comes together, about 30 to 60 seconds.
• Remove the dough from the processor, place it on a clean surface, and knead it for several seconds, until no dry bits remain and the dough is cohesive throughout.
• Preheat the oven to 350°F.
• On a lightly floured surface, roll the dough to ¼ inch thickness. Cut cookies using any cookie cutter shapes you like. Transfer the cookie cutouts to the prepared baking sheets using a spatula.

Bake the cookies.

• Bake the cookies for 8-10 minutes. You'll know the cookies are finished baking when they're no longer shiny and become lightly golden. Let cookies cool completely on the baking sheet atop a wire rack.

Make the vanilla icing.

• Whisk the icing ingredients together in a medium-sized bowl until the mixture is smooth. The mixture should slowly drip, but not run, when poured from a spoon. If the mixture is too runny, add a little bit more sugar. If it’s too thick, add a little bit more milk.

Decorate the cookies.

• When the cookies have cooled completely, get ready to decorate. Spoon the icing into a piping bag or a plastic squeeze bottle and pipe the icing onto the cookies.